Transistor amplifiers having high power handling capability are now used in a wide variety of applications. To provide increased output power, these transistor amplifiers may include transistors having large effective gate peripheries. One technique for increasing the effective gate periphery of a transistor is to provide a plurality of unit cell transistors that are connected in parallel. In such a device, the effective gate periphery may be the sum of the gate peripheries of the individual transistors.
One example type of high power transistor amplifier is the internally matched field effect transistor (“FET”), which is also referred to as an IMFET. An IMFET is a packaged transistor amplifier that includes one or more transistor amplifier integrated circuit chips that may each have a plurality of unit cell transistors that are arranged in parallel to provide a plurality of parallel amplification paths. The unit cell transistors may comprise for example, high electron mobility transistors or “HEMTs” that may be formed using wide bandgap semiconductor materials such as, for example, silicon carbide and/or gallium nitride-based semiconductor materials. The transistor amplifier integrated circuit chip(s) may be packaged in a package together with, for example, other circuit substrates such as printed circuit boards or ceramic circuit substrates that include impedance matching networks, transmission lines, power splitting and combining structures and the like. The package may include one or more input and output leads. Bond wires may be used to inter-connect the integrated circuit chips and other circuit substrates and/or to connect circuit substrates to the input/output leads of the package. IMFET transistor amplifiers may be designed to have operating frequencies that are within specific frequency bands that may fall, for example, between 100 MHz and 28 GHz or even higher frequencies.
Monolithic microwave integrated circuits (“MMIC”) are another type of high power transistor amplifier that includes a plurality of unit cell transistors that are arranged in parallel along with associated matching circuits, feed networks and the like. Like IMFET transistor amplifiers, MMIC transistor amplifiers may include a plurality of unit cell HEMT transistors that are connected in parallel. The primary difference between IMFET and MMIC transistor amplifiers is that in a MMIC transistor amplifier all of the circuit elements of the amplifier—including the transistors, impedance matching networks and feed networks—are formed on a single “monolithic” integrated circuit chip, whereas an IMFET transistor amplifier may have multiple integrated circuit chips and other circuit substrates contained within the packaged device. Bond wires may be used to connect the single integrated circuit chip of a MMIC transistor amplifier to the input/output leads of the protective package.
The individual unit cell transistors in an IMFET or MMIC transistor amplifier may behave slightly differently due to inherent variations in the manufacturing and/or assembly processes used to fabricate the amplifier. While these variations may be subtle, they may be magnified by the impedance matching networks that are typically included in the device. These magnified variations may result in imbalances in phase and/or other parameters between the different legs in the parallel amplification paths, and these imbalances may give rise to oscillations in the output signal. The oscillations may take the form of spurious signals that may be generated within and/or outside of the operating frequency band of the transistor amplifier. The oscillations, whether in-band or out-of-band, may reduce the power of the desired output signal and/or may appear as additional, unwanted signals that can give rise to intermodulation products. In addition, in-band oscillations may appear as noise to the desired output signal.
If the oscillations are large enough, they may seriously degrade the performance of the amplifier. Accordingly, so-called “loop analysis” is performed during the design stage for transistor amplifiers to determine the magnitude and effect of the oscillations. If the loop analysis indicates that the oscillations are sufficiently large, a designer may then add series and/or shunt resistances along the parallel amplification paths through the unit cell transistors in order to stabilize the imbalances in order to reduce the oscillations in the output signal. Unfortunately, the inclusion of such resistances lowers both the gain and efficiency of the amplifier, and may also lower the output power level of the device, which are three of the key performance parameters for IMFET and MMIC power amplifiers.